1. Field of the Invention
This invention relates to emitters having conversion materials, and in particular to solid state emitters having multiple conversion materials to compensate for emitter wavelength variations.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and extracted to the surrounding ambient from all transparent surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material “downconverts” the wavelength of some of the LED's blue light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
Current white LEDs are fabricated in packages by selecting the phosphor or down-conversion material based on their emission properties alone. Preferably, the phosphor or converter material provides a broad and flat excitation spectrum to achieve a constant brightness of phosphor emission even if the LED wavelength is changing. The result is the Lumen output of the white LED is relatively constant independent of the blue LED emission wavelength.
These packages, although fabricated pursuant to controlled methods and standards, can experience variations of the color or hue of the combined light from the LED and the phosphor or converter material. This can be caused by, for example, variations in the amount of phosphor or conversion material covering the LED. In other packages this variation can be caused by other factors such as material composition variations introduced during the LED device fabrication. For example, during the fabrication of gallium nitride based devices, the active region in different devices fabricated from the same or different wafers can have slightly different compositions, such as a varying indium concentration in devices having an indium gallium nitride active region. These different compositions can result in the active region emitting different wavelengths of light. White LED packages and LED systems fabricated using a single phosphor and such blue emitters can suffer from a variation of the color point depending on the LED emission wavelength. These emission variations are difficult to prevent and thus devices or packages utilizing such LEDs in combination with phosphors or converter materials can emit many different colors or hues of white or mixed color light.
The human eye is relatively sensitive to variations in emission wavelengths and white hue, and can detect relatively small differences in emission wavelengths or color. Perceptible variations in color emitted by packages designed to emit a single color of light can reduce customer satisfaction and reduce overall acceptance of LED packages for commercial uses. To ensure that the LEDs provided to the end customer emits light within an acceptable color range, LEDs can be tested and sorted by color or brightness into different bins, generally referred to in the art as binning. Each bin typically contains LEDs from one color and brightness group and is typically identified by a bin code. White emitting LEDs can be sorted by chromaticity (color) and luminous flux (brightness). Color LEDs can be sorted by dominant wavelength (color) and luminous flux (brightness), or in the case of certain colors such as royal blue, by radiant flux (brightness). LEDs can be shipped, such as on reels, containing LEDs from one bin and are labeled with the appropriate bin code.
FIG. 1 shows one example of binning chromaticity regions plotted on the 1931 CIE Coordinate system for commercially available white emitting Cree® XLamp® XR-E and XR-C LEDs provided by Cree Inc. Each of these regions corresponds to a particular chromaticity range of white LEDs with the regions shown surrounding the black body curve or black body locus (BBL). Each of these regions is designed to designate chromaticity variations that are within acceptable ranges to the human eye. For example, region WF designates a particular region having substantially imperceptible chromaticity variations such that LEDs emitting within this region would be binned together.
This binning process typically lowers the product yield and increases the manufacturing cost of LEDs by the overhead associated with the testing and separation of devices with different emission characteristics, and the formulation of the data and records surrounding this process. The greater the number of bins for a particular LED being manufactured, the greater the additional cost associated with the binning process. This in turn can result in increased cost for the LEDs and related packages.